Amplifier gain control circuit



Sept. 14, 1937. A. H. COOPER 2,093,072

AMPLIFIER GAIN CONTROL CIRCUIT Filed Sept. 15, 1953 gal/A65 E H M MAW 05x lNVENTOR ATTORNEY Patented Sept. 14, 1937 UNITED STATES AMPLIFIER GAIN CONTROL CIRCUIT Arthur H. Cooper, Iver, England, assignor to Electric & Musical Industries, Ltd., Middlesex, England, a corporation of England Application September 15, 1933, Serial No. 689,518 In Great Britain October 3, 1932 4 Claims. (01. 179-171) The present invention relates to amplifier circuits, and more particularly to a gain control arrangement for an amplifier.

It is known to bias the two valves of a push- 5 pull amplifier in such a way that each valve handles both the positive and the negative half cycles of the wave to be amplified. The bias has usually been made such that both valves operate on the relatively straight portions of their characteristic curves. With such arrangements the grid voltage swing which can be handled without moving on to the curved part of the characteristics is no greater than with a single valve. In order to increase the power handling capacity of such amplifiers, the valves have been biased at the lower ends of the straight portions of their characteristic curves and during each half cycle of grid voltage one of the valves is operative on the straight part of its characteristic and the other is substantially inoperative.

Due to the curvature of the lower parts of the characteristics between the point corresponding to zero alternating grid voltage and the point corresponding to zero anode current, however, the valve which should be inoperative over the whole of one half swing is in fact operative over the earlier part of this half swing and in a nonlinear manner. The distortion thus introduced may not be serious so long as the grid voltage swing is relatively large, but at small voltage swings, corresponding to low intensities, the nonlinear part of the swing may constitute an appreciable fraction of the whole swing and may become objectionable. To reduce such' distortion, therefore, it has been proposed to associate with the grid circuits of the valves, auxiliary rectifiers which render each valve unresponsive to one half swing. Unless such rectifiers are themselves linear, however, distortion will not be removed.

It has further been proposed to bias the valves of a push-pull amplifier at the lower ends of the straight portions of their characteristics, and to provide means whereby the efiective bias voltage increases (becomes less negative) as the alternating voltage applied to the grids increases. In line transmission systems it has sometimes been found desirable to reduce the variations in energy level of a signal (for example a musical program) to be transmitted and to restore the original extent of variation of level at the receiving station. For this purpose there has been, in effect, shunted across the line at the receiving station a pair of valves arranged in push-pull relationship. These valves have not been used as amplifiers but they have been arranged in such a. way that their impedance increases automatically, and less of the signal is therefore shunted away, as the energy level of the received signal increases. This has been done with the aid of a rectifier device arranged in the input circuit to the valves.

According to the present invention there is provided a thermionic valve amplifier specifically comprising two amplifying valves arranged in push-pull relationship, and means whereby the negative bias applied to the control grids of the valves is automatically increased as the energy level or amplitude of the oscillations to be amplified increases.

Further according to this invention there is provided an amplifier comprising two thermionic .val'ves arranged in' push-pull relationship and having means operating automatically whereby, when the energy level or amplitude of the oscillations to be amplified is low, the control grids of the valves are so biased that the valves operate on the relatively straight portions of their characteristic curves, but that, as the said energy level or amplitude rises, the bias of the control grids is altered in such a way that the valves operate nearer the lower bends of their characteristic curves. In this way the output of given valves at a given high tension voltage can be greatly increased without introducing distortion at low levels of intensity. I

In describing the specific mode of carrying the invention into effect, reference is made to the accompanying drawing wherein two amplifying valves l and 2 are shown arranged in push-pull with the primary Winding 3 of an output transformer T connected between their anodes.

positive terminal of a source Bof high tension current, which is usually a rectifier and filter operated from alternating current mains. The negative terminal of the source B is connected to ground, and through a resistance R, to the filaments of the Valves. This resistance serves to furnish a negative bias to the grids of the valves, and is of such magnitude that the valves are normally biased to about the middle of their characteristic curves. The bias should preferably be as low as possible consistent with reasonable anode circuit dissipation.

The anode of one of the valves 2 is connected through a condenser 4 to the anode 5 of a diode rectifier B, the filament 1 thereof being grounded through resistor R. The anode of the diode is connected to ground through two resistances R2 and R3 arranged in series. The junction point 9 of resistors R3 and R2 is connected to ground through condenser 8, and the latter is, therefore, in shunt with resistor R3. The junction point 9 of the two resistances R2 and R3 is also connected to the center point of the secondary winding ll] of an input, or intervalve, transformer II,

the ends of this winding being connected to the grids of the two valves. Heating current is supplied in any known or suitable manner to the filaments of the two push-pull amplifiers and. the diode.

The center point of this winding is connected to the Amplified signal energy appearing in the output circuit of tube 2 is impressed on the diode 6 through condenser 4, and the energy is rectified and applied to the grids of tubes l and 2. This follows from that fact that the diode anode 5 is connected to the diode cathode 1 through a path including resistor R2; resistor R3; the grounded terminal of the B supply source; resistor R; the common cathode connection of tubes l and 2; and the lead between the'cathode side of condenser l2 and the cathode 'i of diode 6. The grids of tubes I and 2 are connected to resistor R. through resistor R3. In the absence of signals having the required amplitude the grids of tubes I and 2 are biased by the voltage drop across resistor R; when signals in the output circuit of tube 2 exceed the normal negative bias on diode anode 5 then the grids assume an increased negative bias which is dependent upon the voltage drop across resistors R and R3.

The effect of the arrangement above described is that as the anode current swing increases, the negative bias applied to the grids of theamplifying valves increases. The arrangement may be made such that with maximum anode current swing the grid bias reaches a value on the characteristic curve corresponding to substantially zero anode current. Many modifications of the above described arrangement are possible within the scope of the present invention.

With the circuit arrangement described, the load on the high tension source remains fairly constant. Thus, where the arrangement described forms the output stage of a multi-stage amplifier, for example, the problem of potential supply is simplified because of the reduction in feed back to the earlier stages of the amplifier. In an amplifier comprising two valves each of internal impedance approximately 850 ohms and amplification factor .5 arranged in push-pull, the following values of components were found to give good results. A condenser 4 of capacity 0.1 microfarad was connected between the anode of one of the valves 2 of the push-pull pair and the anode of the diode rectifier. 6, the latter being grounded through the two resistances R2 and R3, each of the value 0.1 megohm, in series. Of these resistances, theone nearen ground was shunted by a condenser 8 of capacity 2.0 microfarads, and a bias resistance R of value 500 ohms was connected between the cathodes of the pushpull pair and the negative terminal of the source B. A condenser l2, of 0.1 microfarad, is connected between the cathodes of tubes 1 and 2 and the midpoint of winding l0.

In a modification, a resistance 4' is connected between the anode of one of the amplifying valves 2, and the condenser 4 of which one pole is connected to the anode of the diode. This resistance, which, in the amplifier particularly described above, was given a value of 0.1 megohm, serves to restrict the by-passing of the higher audio frequencies to ground through the diode coupling condenser. The range of additional grid bias voltages applied to the grids of the amplifying valves is determined by the values of the diode shunting resistances and the value of the resistance or resistances mentioned above. All of these resistances may therefore be given such values as to provide the desired range of additional grid bias volt-ages, consistent with the minimum by-passing of high audio frequencies.

What is claimed is:- 1. In a signal amplifier circuit including a tube having signal input and output circuits, said tube being provided with at least a grid, cathode and output electrode, a diode connected to said output circuit for rectification of amplified signals, a resistor in the space current path of said tube for developing thereacross a direct current voltage derived from space current flow therethrough, connections from the said resistor to the grid and cathode to establish the grid at a desired negative bias with respect to the cathode, and said resistor being included in the diode space current path whereby rectified signal current flowing through said resistor develops additional negative bias for said grid.

2. In a circuit as defined in claim 1, said resistor being connected between the diode electrodes in series with an impedance, and the grid and cathode being connected across said resistor and impedance, the diode space current flowing through the latter two providing additional negative bias for the grid.

3. In an amplifier, a pair of tubes arranged in push-pull relation, a source of signals connected to the common grid circuit of the tubes, an output circuit coupled to the anodes of the tubes, an impedance in the space current paths of the tubes, a bias connection between the said grids and said impedance, said impedance having a magnitude such that the grids are biased, for signals below a predetermined amplitude, to permit the tubes to operate on the relatively linear portions of their. grid voltage-plate current characteristics, a space current device including at least an anode and cathode, a portion of said bias connection, including said impedance, being connected between the anode and cathode of said device, signal connections between the said output circuit and the device, and Said last named portion having a magnitude such that the bias of said grids is altered with increasing signal amplitude, in a sense to permit the tubes to operate adjacentthe lower bends'of said characteristics.

4. In a signal amplifier circuit of the type provided with a tube having at least an input electrode, a cathode and an output electrode, a signal input circuit coupled between the input electrode and cathode, an output circuit coupled between the output electrode and cathode, an impedance in the space current path of the tube developing a direct current voltage from the space current, means applying the voltage between the cathode and input electrode in a sense to establish the latter at a negative potential, a diode means applying saidvoltage tothe diode electrodes in a sense to normally establish the diode 

